1. Field of the Invention
The present invention relates to devices for the registration of drops in infusion devices, and in particular to such an apparatus wherein the infusion device includes a drip chamber connected to a reservoir for supplying infusion fluid to a patient via a tube.
2. Description of the Prior Art
Various factors complicate registration of a drop in a drip chamber when optical measuring methods are utilized in an infusion device of the type having a reservoir containing infusion fluid which is supplied to the drip chamber, the fluid being then supplied from the drip chamber to the patient via a tube. Optical measuring methods employing a light source and a light sensor may give false data when the transparency of the wall of the drip chamber becomes deteriorated such as, for example, by fluid vapor precipitation thereon, splattering resulting from drops incident on the fluid surface within the chamber, or jolts to the drip chamber. Splatters or a rippled fluid surface caused by vibrations can additionally cause incorrect drop indications. Vibrations or tilting of the drip chamber may mean that the drop descends obliquely through the chamber. It is also possible for the electronics employed for processing the signals or the light source or the light sensor, to deteriorate over time in terms of electrical properties, and thereby render the registration less certain.
One apparatus attempting to avoid such errors in drop registration is disclosed in German OS No. 28 30 512, wherein a plurality of light sources and photodetectors are disposed on the circumference of the drop chamber in a plane perpendicular to the drop direction. This apparatus still provides reliable registration data even if the drip chamber is unfavorably tilted.
Another apparatus is known from U.S. Pat. No. 4,038,981 wherein one light source and two phototransistors are disposed in one plane. The sensitivity of the drop sensor and tilted attitudes of the chamber are taken into consideration by this device. A fluid line is released by a clock generator and an error element is set at the same time, the error element triggering a fault indication and/or an alarm when a reset signal is not received from the drop detector within a predetermined time. In this apparatus, a drop must be released each time by means of an actuating valve, the occurrence of the drop being documented by the drop sensor, whereupon the valve is then closed. The phototransistors are connected in parallel, with their collector voltages switched to a pulse shaver by an ac voltage signal. The circuit is thus balanced even when less light is incident on the phototransistors. When, however, the sensitivity of the apparatus becomes so poor for any reason such that drop detection is uncertain, this may result in serious errors. It may occur, for example, that only every other drop is registered and thus an undesirably large amount of infusion fluid is administered to the patient. Additionally, a measurement of blood pressure may, for example, delay the drop initiation and trigger an alarm by the error stage when no such alarm is actually warranted.